Project Summary Frontotemporal lobar degeneration (FTLD) encompasses a group of neurodegenerative disorders characterized by cognitive and behavioral impairments. Heterozygous mutations in progranulin (PGRN) result in decreased PGRN expression and account for ~25% of familial FTLD. In contrast, homozygous PGRN mutations result in complete loss of PGRN and lead to neuronal ceroid lipofuscinosis (NCL), a group of neurodegenerative lysosomal storage disorders. Thus, PGRN mutations appear to cause different diseases (FTLD vs NCL) in a dose-dependent manner, suggesting that heterozygous PGRN mutations might cause FTLD via partial loss of lysosomal function. My PhD Dissertation Project aims to determine if reduced PGRN expression, due to FTLD-linked PGRN heterozygous mutations, causes lysosomal dysfunction and contributes to neurodegeneration in FTLD. Using iPSC-derived human cortical neurons derived from FTLD patients harboring PGRN mutations compared to isogenic controls, we have demonstrated that PGRN mutant neurons have significantly impaired lysosomal proteolysis. To determine the mechanism of this impaired lysosomal proteolysis, we examined the relationship between PGRN and the lysosomal enzyme cathepsin D. Mutations in PGRN and CTSD both lead to similar forms of NCL, and cathepsin D is predominantly expressed in the brain where it is responsible for the degradation of long-lived proteins. We found that cathepsin D activity, but not its expression was significantly decreased in PGRN mutant neurons. Furthermore, we demonstrated that PGRN interacts with cathepsin D, and that granulins, cleavage products of PGRN, significantly increase cathepsin D activity in vitro. Based upon these initial results, we propose a novel role for PGRN in regulating lysosomal cathepsin D activity, which is disrupted by loss of PGRN expression in FTLD-linked heterozygous mutant neurons, leading to defective lysosomal function in FTLD. To further investigate these results, we will perform in vitro dose-dependent cathepsin D activity curves using recombinant granulins to determine if individual granulins cleaved from PGRN specifically regulate cathepsin D activity. Furthermore, we will determine if PGRN interacts with or alters the activity of other lysosomal enzymes in addition to cathepsin D. Finally, we will use long-term cultures of FTLD patient-derived PGRN mutant iPSC cortical neurons to determine if lysosomal dysfunction resulting from decreased cathepsin D activity contributes to pathogenic FTLD hallmarks such as ubiquitin and TDP-43 positive inclusion formation. This PhD Dissertation Project will provide important insight into both the normal role of PGRN in regulating lysosomal function and the cellular mechanisms by which PGRN mutations cause FTLD in human neurons. As a Postdoctoral trainee, I will expand upon my PhD training by studying how glial cells contribute to neurodegenerative phenotypes. Ultimately, I plan to merge my PhD and postdoctoral training by becoming an academic researcher investigating the role of neuronal-glial interactions in the pathogenesis of neurodegeneration.